LT8708
45
Rev 0
when IMON_INP and IMON_INN, respectively, are used
to operate the LT8708 at constant current limit.
INTV
CC
REGULATORS AND EXTV
CC
CONNECTION
The LT8708 features two PNP LDOs (low dropout regu-
lators) that regulate the 6.35V (typical) INTV
CC
pin from
either the V
INCHIP
or EXTV
CC
supply pin. INTV
CC
powers
the MOSFET gate drivers via the required GATEV
CC
connec-
tion and also powers the LDO33 pin regulator and much
of the LT8708’s internal control circuitry. The INTV
CC
LDO
selection is determined automatically by the EXTV
CC
pin
voltage. When EXTV
CC
is lower than 6.2V (typical), INTV
CC
is regulated from the V
INCHIP
pin LDO. After EXTV
CC
rises
above 6.4V (typical), INTV
CC
is regulated by the EXTV
CC
pin LDO instead.
Overcurrent protection circuitry typically limits the
maximum current draw from either LDO to 127mA. When
GATEV
CC
and INTV
CC
are below 4.65V, during start-up or
during an overload condition, the typical current limit is
reduced to 42mA. The INTV
CC
pin must be bypassed to
ground with a minimum 4.7μF ceramic capacitor placed
as close as possible to the INTV
CC
and GND pins. An ad-
ditional ceramic capacitor should be placed as close as
possible to the GATEV
CC
and GND pins to provide good
bypassing to supply the high transient current required by
the MOSFET gate drivers. 1μF to 4.7μF is recommended.
Power dissipated in the INTV
CC
LDOs must be minimized to
improve efficiency and prevent overheating of the LT8708.
Since LDO power dissipation is proportional to the supply
voltage and V
INCHIP
can be as high as 80V in some applica-
tions, the EXTV
CC
pin is available to regulate INTV
CC
from
a lower supply voltage. The EXTV
CC
pin is connected to
V
OUT
in many applications since V
OUT
is often regulated to
a much lower voltage than the maximum V
INCHIP
. During
start-up, power for the MOSFET drivers, control circuits
and the LDO33 pin is usually derived from V
INCHIP
until
V
OUT
/EXTV
CC
rises above 6.4V, after which the power is
derived from V
OUT
/EXTV
CC
. This works well, for example,
in a case where V
OUT
is regulated to 12V and the maximum
V
INCHIP
voltage is 40V. EXTV
CC
can be floated or grounded
when not in use or can also be connected to an external
power supply if available.
The following list summarizes the three possible connec-
tions for EXTV
CC
:
1. EXTV
CC
left open (or grounded). This will cause INTV
CC
to be powered from V
INCHIP
through the internal 6.3V
regulator at the cost of a small efficiency penalty.
2. EXTV
CC
connected directly to V
OUT
(V
OUT
> 6.4V). This
is the normal connection for the regulator and usually
provides the highest efficiency.
3. EXTV
CC
connected to an external supply. If an external
supply is available greater than 6.4V (typical) it may
be used to power EXTV
CC
.
Powering INTV
CC
from EXTV
CC
can also provide enough
gate drive when V
INCHIP
drops as low as 2.8V. This allows
the part to operate with a reduced V
INCHIP
voltage after
V
OUT
gets into regulation.
The maximum current drawn through the INTV
CC
LDO
occurs under the following conditions:
1. Large (capacitive) MOSFETs are being driven at high
frequencies.
2. V
IN
and/or V
OUT
is high, thus requiring more charge
to turn the MOSFET gates on and off.
3. The LDO33 pin output current is high.
4. In some applications, LDO current draw is maximum
when the part is operating in the buck-boost region
where V
IN
is close to V
OUT
since all four MOSFETs are
switching.
To check for overheating find the operating conditions that
consume the most power in the LT8708 (P
LT8708
). This
will often be under the same conditions just listed that
maximize LDO current. Under these conditions monitor
the CLKOUT pin duty cycle to measure the approximate die
temperature. See the Junction Temperature Measurement
section for more information.
LDO33 REGULATOR
The LT8708 includes a low dropout regulator (LDO) to
regulate the LDO33 pin to 3.3V. This pin can be used to
power external circuitry such as a microcontroller or other
desired peripherals. The input supply for the LDO33 pin
APPLICATIONS INFORMATION
